The Preface explains this style as Merlyn writing his memoir of how he met Arthur and came to raise him.
Saxon Shore begins with Merlyn and the infant Arthur stranded in a small boat on the southern most extreme of the Irish Sea. An Irish pirate ship captained by Connor, a prince of Eire, captures the boat. The Celts then throw the child overboard. Disregarding his life, Merlyn kills one of the pirates and jumps in after Arthur. The pirates recapture Merlyn and the floating child and return them to the ship. The captain greets Merlyn and the child, revealing the origin of the crew, Eire, and tries to understand the reasons why Merlyn would sacrifice his life for the child. In the conversation, the captain comes to realize that his brother, Donuil is Merlyn's captive at Camulod, so he releases Merlyn in the agreement that the child will be returned if his brother returns to Eire.
Merlyn then proceeds home, where he quickly becomes embroiled in factions politics that have arisen in the Camulodian council. By using his military authority and appealing to the older council, Merlyn disbands the parties. Ironhair, one of the faction leaders, becomes enraged by this and makes an assassination attempt on Merlyn's life. Meanwhile, Donuil returns to Camulod with Merlyn's nearly identical half-brother Ambrose. Once Donuil returns, Merlyn creates a party which is to escort Donuil back to Eire.
On the trip to Eire the party has encounters with a leper colony, where Lucanus, a physician and Merlyn's longtime friend, leaves the party to deliver a wagon-load of supplies to the impoverished lepers. When the party arrives there, a crew of marauders was harvesting marble from a variety of buildings in Glevum: a Roman temple, and a large and impressive administrative basilica and forum market-place. Merlyn decides that they will be unable to gain passage on any ships there, after a brief skirmish with the locals.
After the encounter with the scavengers, a group of Scots sent by Donuil's father to ensure his safe return find the party. Soon the two galleys of the Scots are hauling a barge to Eire where the barge capsizes south of Athol's kingdom. The Comuludian knights travel through the Irish wilderness under threat of barbaric peoples, but only encounter a boar larger than any other ever hunted by the Scots. Within several weeks of leaving Comulud the party arrives in the capital of Athol's kingdom.
The party stays at the stronghold and Merlyn, in conversation with Athol, reveals that he was married to one of Athol's daughters, Deirdre. Athol accepts Merlyn into his family. During the same conversation Merlyn also reveals the identity of the child, Arthur, and Athol pledges himself as an ally to Merlyn and his Grandson. While staying in the stronghold of the Scots, Merlyn and his men demonstrate the use of cavalry to the Scots who had previously never seen its use in battle. During the exhibition a bear enters the clearing and attacks. Merlyn uses his memory of Alexander the Great's bodyguard using Sarissa, heavy lances, to charge troops, acquiring a spear from infantry that were to be part of the demonstration and charging the bear.
One evening, a member of the community disappears and, while searching for said man, Donuil feels that someone was watching in the woods. Merlyn's retinue and Athol's warriors are put on alert, and in the morning an army attacks the walls. The strength of the cavalry successfully routes the attacking army in two charges. The attack of the wild men of the south is an unruly advance force of the eminent attack by the MacNyalls, Sons of Condran, and Sons of Garn. Athol decides that Merlyn, Arthur, Donuil and their company must return to Briton to avoid this attack and ensure Arthur's safety.
The Saxon Shore
The party of Merlyn returns to Camulod without Donuil, who returns to Eire in order to stave off the events of one of Merlyn's dreams. While traveling back to Camulod the party encounters a group of marauding Berbers. Upon returning to Camulod, Merlyn discovers his half brother Ambrose has integrated the infantry and cavalry in order to reduce enmity between the two military branches. A group of Cambrian raid an outlying farm of the colony, however before the military can follow a heavy winter sets in that kills the oldest members of the community.
In the spring a large contingent of the military, 500 foot soldiers and 500 cavalry, leave Camulod to take revenge for the raid which killed 50 of their comrades. Led by Merlyn, the force travels near the leper colony that was visited with Lucanus and the whole colony is found dead. The military force also clears the Berbers from their pirate outpost in Glevum. The army enters Cambria and soon find the men who had stolen the horses dead, they then encounter a force of Dergyll's archers, however Merlyn tactfully avoids any confrontation. The two leaders agree to an alliance and in proof of their loyalty to the alliance, they exchange a small contingent of auxiliary forces. While discussing this Merlyn discovers that Ironhair, who had led one of the political parties in Camulod, was now supporting a contender for the Pendragon throne.
Merlyn and the forces return to Camulod and years of peace ensue. Merlyn and Ambrose make a trip to Northumberland and discover that the alliance between Briton-Romans and the Norse that had maintained the strength of the kingdom is failing. They return to Britain and begin the education of Arthur, along with the other family and friends of Merlyn. An attempt is made on Arthur's life by a group of men loyal to Ironhair, and the council of friends which had come to surround Merlyn decided that in order to protect this future king he must live outside of the community which knows of his existence. Merlyn decides to settle Arthur in Ravenglass south of Hadrian's Wall.
Arthur and Merlyn travel to Ravenglass aboard Connor's galley and are welcomed by the Ravenglass King Derek.